The rigid structure of cell walls of plant tissues is due to xylans together with other hemicelluloses, pectins, cellulose and lignin. Xylans form the major hemicellulose, most xylans are heteropolysaccharides with a homopolymeric backbone chain of 1,4-linked .beta.-D-xylopyranose units. The plant of origin determines the degree and the type of substitutions of the specific xylan. Xylans are found to contain many different side groups, among these L-arabinose, D-glucuronic acid or its 4-O-methyl ether, and acetic, p-coumaric, and ferulic acids are the most prominent.
It has been suggested that both acetyl and arabinosyl substituents increase the solubility of hemicellulose by decreasing the possibility of intermolecular aggregation, however, these substituents are at the same time a severe hindrance to the enzymatic degradation of the plant tissues. For example, it has been reported that acetylation inhibits the digestibility of plant polysaccharides in ruminants. Poutanen and Puls (1989) (In Biogenesis and Biodegradation of Plant Cell Wall Polymers (Lewis, N. and Paice, M. eds) ACS Symp. Ser. 399: 630-640), have shown that the major xylanase of Trichoderma reesei is unable to depolymerize acetylated soluble xylan. Grohmann et al. (1989) (Appl. Biochem. Biotechnol. 20/21: 45-61) have shown that after chemical deacetylation xylan is 5-7 times more digestible by ruminants.
Esterases (EC 3.1.1.6) are classified according to their substrate specificity. Since it is generally difficult to determine the natural substrate for these enzymes the classification is problematic and this problem is enlarged by the widespread appearence of esterases in nature. It is therefore not surprising that although the existence of enzymes that deacetylate xylan may have been anticipated in view of the long known occurrence of microbial esterases that were known to act on various synthetic substrates, it was not until recently that the existence of acetyl xylan esterases was demonstrated.
Biely et al. (1985, FEBS Lett. 186: 80-84) demonstrated the presence of acetyl xylan esterases in (fungal) cellulolytic and hemicellulolytic systems: Trichoderma reesei, Aspergillus niger, Schizophyllum commune and Aureobasidium pullulans. As compared with plant and animal esterases, these fungal esterases exhibit high specific activities towards acetylated glucuronoxylan and were therefore named acetyl xylan esterases.
Further investigations on the fungal acetyl esterases have been reported. Poutanen et al. (1988, Appl. Microbiol. Biotechnol. 28: 419-425 and 1990, Appl. Microbiol. Biotechnol. 33: 506-510) described the purification and characterization of acetyl xylan esterases from T. reesei. Enzymatic deacetylation of xylan using purified acetyl xylan esterase resulted in the precipitation of the remaining polymer structure. Due to this effect acetyl esterase is not used as a single first enzyme in the degradation of acetylated xylans. The highest xylose yield from acetylated xylan was obtained by the synergistic action of xylanase, .beta.-xylosidase and acetyl xylan esterase.
To achieve a practically useful degradation of xylans there is a need for large amounts of the enzymes involved in the enzymatic hydrolysis of these highly substituted molecules. The present invention provides a way for obtaining large amounts of fungal acetyl xylan esterases, optionally in a purified form.